1. Field of the Invention
This invention relates to rubber sheets, as used in the manufacture of power transmission belts and, more particularly, to rubber sheets containing short reinforcing fibers. The invention is directed to both a method and an apparatus for manufacturing such rubber sheets.
2. Background Art
It is known to form one or more layers of a power transmission belt from rubber with short reinforcing fibers embedded therein. It is known to mix short fibers with an unvulcanized rubber and to place the unvulcanized rubber with the fibers therein between a pair of calendar rolls operating at different rotational speeds. Through this mechanism, the lengths of the fibers orient substantially in the direction of advancement of the rubber through the cooperating rolls. The resulting sheet is then cut to a width, dictated by the particular belt configuration into which it is to be incorporated, and placed in a roll form. The sheet and one or more like sheets and/or other components are laminated to produce a desired thickness. The fibers in the sheets are oriented so that their lengths extend widthwise of a belt into which the sheets are incorporated, to increase the lateral stability thereof.
The components/layers of the belts are conventionally serially wrapped upon a drum during the manufacturing process. More particularly, a conventional manufacturing process for a V-ribbed power transmission belt, or a raw-edge V-belt, is carried out as follows. One or more sheets of a covering canvas and cushion ruber layer are wrapped onto the outer surface of a cylindrical molding drum. Load carrying cords are spirally wrapped around the cushion rubber layers. One or more compression rubber layers are wrapped in turn to complete a belt sleeve preform, which is vulcanized. The compression section of the belt may typically have a thickness defined by three to four such laminated layers. One or more layers of the sheets with the fibers embedded therein may be wound around the molding drum to produce a desired thickness in the compression section, or elsewhere.
As a practical matter, in order to consistently orient the short fibers using the process described above, the sheets must be relatively thin. This may ultimately require the superimposition of numerous sheets to produce the desired thickness for a particular sheet/layer. As a result, the manufacturing process may be complicated by having to laminate multiple sheets/layers of the rubber material having the short fibers embedded therein.
To avoid this problem, it is known to form rubber sheets of substantially greater thickness with consistently oriented short fibers using an extruding process employing an expansion die. An exemplary expansion die is shown in Japanese Examined Patent Application Publication No. 9847/1994. The expansion die therein has an annular expansion space with a diameter that increases between inlet and discharge locations. The width of the space also varies between the inlet and discharge locations. More particularly, the width at the entry location is narrower than at an intermediate location. The width at the discharge location is less than the width at the intermediate location and greater than the width at the entry location.
Japanese Unexamined Patent Application Publication No. 106602/1994 discloses an apparatus in which a cutter severs a cylindrical elastomer shape containing short fibers oriented in the circumferential direction continuously as the cylindrical shape discharges from a die. With this mechanism, an axial cut is continuously formed so as to allow the cylindrical shape to be formed into a flat sheet. Air is blown at the cylindrical shape between the die and the cutter so as to effect cooling thereof. This cooling inhibits shrinkage of the cylindrical shape in a circumferential direction as might disturb the orientation of the fibers that may occur as a result of non-uniform shrinkage. The problem of flaring is avoided by maintaining equal distances between the cut ends of the sheet and the center thereof.
In this conventional expansion die arrangement, the orientation ratio is three-dimensionally adjusted by a) controlling the space width ratios between the entry location and intermediate location and the intermediate location and discharge location, b) the radii of the entry and discharge locations, and c) the sectional areas. Because the orientation ratio is controlled by flow pressure, as dictated by the shape of the die, the orientation ratio can be changed only by changing the dies.
In the above-described prior art structure, flow of rubber is diverted from horizontal to vertical between a feeding cylinder and the expansion die. By reason of the orientation of the axis of the expansion die in a vertical direction, internal strain, such as deflection, can be made relatively small. Resultingly, the thickness of the cylindrical shape that is extruded in the expansion die can be consistently maintained. However, the construction described above has some drawbacks.
The prior art uses a cross head design which splits the incoming flow to the expansion die to effect uniform circumferential distribution of the material. Accordingly, a weld line is formed at the point of diversion. The split flow converges at a second weld line at a diametrically opposite location. The converging flows at the second weld line produce a random orientation of the fibers thereat and potentially a non-uniform thickness.
Preferably, the discharging cylindrical shape will be axially cut at the weld lines and thereafter reshaped into a sheet which is wrapped around a roll A, as shown in FIG. 5. The edges B, B′ in FIG. 5, bounding the cut produced at C, expand at an angle a, a′ to vertical, between the point of incision at C and the location where the roll A is located. Accordingly, portions of the cylindrical shape are significantly stretched vertically to facilitate the rolling shown in FIG. 5, whereas other portions, such as those diametrically opposite to the incision location, are not vertically stretched to any significant extent. It has been found that when the lengths D, D′ of the edges B, B′, between the incision location and the locations E, E′ at which the edges B, B′ engage the roll A, exceed a certain value, the elongation of the cylindrical shape may remain as a permanent strain, thus causing a flaring phenomenon upon the finished sheet.